The present invention relates to an image reading unit and an image reading apparatus used as a scanner and the like installed in a facsimile device, an image forming apparatus, and a computer.
An image reading apparatus includes an image reading unit having an image sensor such as a CCD mounted in a box shaped casing. The image reading unit is moved in a sub-scanning direction to read an original mounted on a platen fixed to a casing. In the casing, there are provided a driving source such as a motor for driving the image reading unit, a timing belt for transmitting a drive from the driving source to the image reading unit, and a driving system such as a pair of pulleys on which the timing belt is extended.
In recent years, in order to reduce a weight of the apparatus and the number of parts, the casing has been integrally formed with the apparatus. Accordingly, there may be a case that the casing does not have enough rigidity, thereby making it difficult to perform a reading scanning operation precisely. Further, since the casing expands or contracts due to a temperature change and a distance between the pulleys changes, the timing belt is tensioned with various forces and may skip teeth of the pulleys. Accordingly, it is difficult to perform a precise scanning operation to obtain an image with good quality.
Japanese Patent Publication (Kokai) No. 2001-238012 has disclosed an image reading apparatus in which four reinforcing plates, i.e. two reinforcing plates extending along a sub-scanning direction and two reinforcing plates extending along a main scanning direction, are attached to an upper surface of a box shape casing for reinforcing the casing formed of a resin. In the image reading apparatus, one of the reinforcing plates extending along the sub-scanning direction is used also as a guide member for guiding an image reading unit, thereby reducing the number of parts and improving assembly efficiency.
In the technique disclosed in Japanese Patent Publication (Kokai) No. 2001-238012, after components of a driving system are individually fixed to the casing with screws, a distance between pulleys is adjusted, thereby deteriorating the assembly efficiency and productivity. Further, it is difficult to improve scanning accuracy of the image reading unit against a temperature change.
In the image reading apparatus described above, the image reading unit includes a frame made of a resin; a light source attached to the frame for irradiating an original; a reflection unit having a plurality of mirrors for reflecting the light reflected from the original; a lens unit for forming an image of the reflected light from the original through the reflection unit; and an image sensor disposed at a position where the lens unit forms the image and having a plurality of photoelectric transfer elements such as CCDs (Charge Coupled Devices) arranged in a line. After the reflected light (image light) from the original is reflected through the reflection unit, the lens unit forms the image on the image sensor. The formed image is converted into an electric signal, and the electric signal is converted into a digital signal to be output.
In general, a frame of an image reading unit is formed of a resin to reduce a weight and cost. Japanese Patent Publication (Kokai) No. 05-30292 has disclosed a configuration in which a frame is provided with openings in sidewalls thereof for attaching mirrors, and the mirrors are directly attached to the frame with fixing fittings pressing ends of the mirrors against the openings.
In the technique disclosed in Japanese Patent Publication (Kokai) No. 05-30292, the frame thermally expands or lowers strength due to heat of a light source, so that the mirrors are shifted from the attached positions, thereby making it difficult to obtain a good image.
To solve such a problem, a pair of metal plates may be attached to sidewalls of a frame made of a resin, and mirrors are fixed to the plates. Accordingly, because of a low coefficient of thermal expansion and a high strength of the metal, it is possible to reduce a shift of the mirrors from attached positions, thereby obtaining a good image and reducing a weight and cost.
In the case that the mirrors are fixed to the metal plates, the plates are disposed inside the frame. Accordingly, light is irregularly reflected at the plates and incidents on an image sensor, thereby giving an adverse effect on the image. Further, the plates are disposed within a substantially sealed frame. Therefore, when a light source is continuously powered on for a long time in a continuous reading operation, the heat is confined in the frame, so that the plates are undesirably heated up to change the positions of the mirrors, thereby giving an adverse effect on an image.
In view of the problems described above, a first object of the present invention is to provide an image reading apparatus with excellent assembly efficiency capable of performing an accurate scanning operation irrespective of an environmental temperature change and having a casing made of a resin to reduce a weight.
A second object of the present invention is to provide an image reading unit having a frame made of a resin to reduce the weight of the unit, in which change in a position of a mirror is suppressed to a minimum level irrespective of an environmental temperature change to obtain a good image, and an image reading apparatus using the image reading unit.
Further objects and advantages of the invention will be apparent from the following description of the invention.